Add a test against skopeo?

[mtrmac]

So here is a completely horrible idea: As part of tests, perhaps in Makefile and definitely in Travis, checkout skopeo, replace the vendored copy with current working directory, and test that skopeo runs and its tests pass.

As I said, completely horrible. But perhaps worth it, at least until most of the transports in this repo grow some real tests. Merging containers/skopeo#102 and only a week later discovering that it breaks pushing to OpenShift would be pretty awkward…

WDY(all)T?

[runcom]

seems fine to me as a temporary workaround until we figure the whole stuff out and given skopeo is the reference implementation for this library now